Electronic repeaters, wherein a received electromagnetic signal is automatically amplified and then retransmitted, are well known in the art. Use of a repeater enables a relatively low-power original signal, such as that from a mobile telephone unit, to be transmitted with a power orders of magnitude greater than the original signal.
FIG. 1 illustrates a repeater system 10, as is known in the art. A first antenna 12 receives a signal from a first transmitter 13, for example a cellular base-station transceiver system (BTS). The signal is transferred on a coaxial cable 14 to a repeater 16, wherein the signal is amplified and transferred on a coaxial cable 18 to a second antenna 19, which transmits the “repeated” signal generated by repeater 16. Similarly, a signal received by antenna 19 from a second transmitter 15, such as a mobile telephone, traverses a reverse path through system 10, being amplified in repeater 16 and retransmitted by antenna 12. Overall power gains typically required for the signals, from antenna to antenna, are of the order of 90 dB.
Repeaters which separate the functions performed by repeater 16 into two or more separate systems are also known in the art. U.S. Pat. No. 5,404,570, to Charas et al, which is incorporated herein by reference, describes a repeater system used between a base-station transceiver system (BTS) and a closed environment, such as a tunnel, which is closed off to transmissions from the BTS. The system down-converts a high radio-frequency (RF) signal from the BTS to an intermediate frequency (IF) signal, which is then radiated by a cable and an antenna in the closed environment to a receiver therein. The receiver up-converts the IF signal to the original RF signal. Systems described in the patent serve a vehicle moving in a tunnel, so that passengers in the vehicle who would otherwise be cut off from the BTS are able to receive signals.
U.S. Pat. No. 5,603,080, to Kallandar et al., which is incorporated herein by reference, describes a plurality of repeater systems used between a plurality of BTSs and a closed environment, which is closed off to transmissions from the BTSs. Each repeater system down-converts an RF signal from its respective BTS to an IF signal, which is then transferred by a cable in the closed environment to one or more respective receivers therein. Each receiver up-converts the IF signal to the original RF signal. Systems described by the inventors serve a vehicle moving between overlapping regions in a tunnel, each region covered by one of the BTSs via its repeater system.
U.S. Pat. No. 5,765,099, to Georges et al., which is incorporated herein by reference, describes a system and method for transferring an RF signal between two or more regions using a low-bandwidth medium such as twisted-pair cabling. In a first region the RF signal is mixed with a first local oscillator to produce a down-converted IF signal. The IF signal is transferred to a second region via the low-bandwidth medium, wherein the signal is up-converted to the original RF signal using a second local oscillator. The local oscillators are each locked by a phase locked loop (PLL) in each region to generate the same frequency, the locking being performed in each loop by comparing the local oscillator frequency with a single low-frequency stable reference signal generated in one region. The reference signal is transferred between the regions via the low-bandwidth medium.
Cellular communications receivers typically suffer from large variations in received signal power, due to phenomena such as Rayleigh and Ricean fading. The effects of the large variations are exacerbated as a mobile transceiver within the system changes position. A method well-known in the cellular communications art for alleviating the problems is to receive a plurality of signals from the transceiver, typically two signals herein termed main and diversity signals. The two signals are processed in order to enhance the reception from the mobile transceiver. The main and diversity signals are typically received from two respective antennas which in turn receive an initial signal transmitted by the mobile. The antennas may be spatially separated, and/or may receive different polarizations, and/or may have differing time delays introduced into their respective reception paths, generating spatial, polarization, and temporal diversity respectively.